1. Field of the Invention
This invention relates to a power amplifier apparatus to be suitably used for a 4-channel stereophonic system loaded in a motor vehicle.
2. Description of the Related Art
Known power amplifiers that operate with improved efficiency include that disclosed in Japanese Patent Laid-Open Publication No. 6-338738 (JP-A-6-338738) describing a power amplifier adapted for high efficiency operation by selecting a DC output voltage that is very close to the ground potential and driving the load by means of a BTL (balanced transformerless) drive technique, using half-wave signals. FIG. 1 of the accompanying drawings shows a circuit diagram of such a power amplifier.
Referring to FIG. 1, input signal IN is applied to the negative input terminal of a first differential amplifier 1, which produces corresponding output signals with opposite phases from its positive and negative output terminals. The positive and negative output signals of the first differential amplifier 1 are fed to first and second output amplifiers 2 and 3, which amplify the respective signals. Said first and second output amplifiers 2 and 3 constitute a BTL amplifier and the load 4 (e.g., a speaker) of the power amplifier is BTL-driven by output signals X and Y of the first and second output amplifiers 2 and 3.
The output signals X and Y of the first and second output amplifiers 1 and 2 are non-linearly added to each other by means of an non-linear adder 5. The non-linear adder 5 is activated to operate as an adder when the output signals of the first and second output amplifiers 2 and 3 are below a predetermined level, whereas the non-linear adder 5 operates as a clump circuit when the output signals exceed the predetermined level. The output signal of the non-linear adder 5 is applied to the negative input terminal of a second differential amplifier 2, which produces an output signal as a function of the output signal of the non-linear adder 5 and the reference voltage Vref of its positive input terminal. This output signal is then applied to common terminal C of the first differential amplifier 1. The common terminal C is used to determine the output DC voltage of the first and second output amplifiers 2 and 3, which output DC voltage is controlled as a function of the output signals X and Y of the first and second output amplifiers 2 and 3. Therefore, as shown by dotted lines in FIGS. 2(a) and 2(b), the output DC voltage of the first and second output amplifiers 2 and 3 is selected to be close to the ground potential level and the output signals X and Y of the first and second output amplifiers 2 and 3 are half-wave output signals. Thus, the load RL is BTL-driven by the half-wave signals.
Meanwhile, there is also known a 4-channel stereophonic system installed in a motor vehicle and comprising first and second power amplifiers for amplifying left stereophonic signals and third and fourth power amplifiers for amplifying right stereophonic signals, wherein said first and third power amplifiers are paired to drive the front speakers in the cabin of the motor vehicle while said second and fourth power amplifiers are paired to drive the rear speakers in the cabin of the motor vehicle.
A clip detection circuit may be added to a high efficiency amplifier circuit as shown in FIG. 1 in order to detect clips. A clip detection circuit may be so designed that it refers to the input signals of the positive and negative input terminals of the first differential amplifier and determines the occurrence of a clip when the input signals exceed a predetermined level.
When power amplifiers having a circuit configuration as shown in FIG. 1 and comprising a clip detection circuit are used for such a car stereophonic system, a total of four power amplifiers must be installed. If four power amplifiers with the circuit configuration of FIG. 1 are simply combined, then a total of four clip detection circuits must also be used, which makes the entire system bulky and clumsy due to the large number of components. Particularly, if the four power amplifiers are integrally arranged on a single substrate, the chip is inevitably made to show a large surface area mainly due to the existence of clip detection circuits.